A primary goal of this laboratory is to increase our understanding of IL-6-mediated myeloma growth with the ultimate goal of identifying areas which may provide targets for therapeutic intervention. We have performed studies aimed at (1) structural characterization of the IL-6 receptor and (2) analysis of the mechanisms by which these cells escape the requirement for this growth factor. Recent studies in our lab have also revealed that, in addition to autocrine growth, the progression to IL-6-dependence can be mediated by a nonautocrine mechanism. We have gained insight into this mechanism by using somatic cell hybrid experiments in which IL-6-dependence is restored by the introduction of normal (wild type) DNA via cell fusion. The introduction of wild type DNA via cell fusion presumably reconsti- tutes a negative growth/tumor suppressor gene which is normally- regulated by IL-6. Presumably, such a gene would be expected to play a role in IL-6 signal transduction. We are now in the process of identifying this gene. We have created panel of IL-6-dependent rat-mouse somatic cell hybrids. This panel of cells wil allow us to identify the chromosome on which this gene resides and, eventually, to clone the gene responsible for transition to autonomous growth. We have continued the structural characterization of the IL-6 receptor. Although published reports have identified only one IL-6-binding protein, gp80, in the receptor complex, affinity crosslinking studies in our laboratory have identified a 150 kDa crosslinked complex that contains a 130 kDa IL-6-binding membrane glycoprotein. Two other complexes with masses of 100 and 120 kDa are composed gp80 crosslinked to one and two molecules of IL-6, respectively. Our results indicate that (1) the 130 kDa molecule associates directly with IL-6 and (2) the functional receptor comylex may consist of dimers of gp80 plus two molecules of IL-6 and at least one gp130 molecule. We are also pursuing the identity of a protein found crosslinked to IL-6 in a 70 kDa complex. We have established that, in IL-6-dependent murine myeloma cells, the mechanism of death after IL-6 withdrawal is apoptosis. Chronic exposure to phorbol esters (1) delays apoptosis and (2) enhances IL-6-mediated proliferation, thus implicating protein kinase C as a mediator of an anti-growth pathway.